This invention is in the technical field of lithographic projection of a mask pattern on a wafer arranged on a wafer stage and relates in particular to an interferometer system for measuring the height of such a wafer stage for determining an ideal height for illuminating the wafer surface.
U.S. Pat. No. 6,208,407 has described such systems having a projection station and a measuring station. Each wafer, prior to having a pattern being illuminated in the projection station, is brought into a measuring station where not only is the wafer surface mapped to obtain its profile but also the height is measured of the stage on which the wafer is transported from the measuring station into the projection station and is exposed to a projection beam in the projection station. Wafer height sensors for this purpose have also been described in U.S. Pat. No. 6,208,407, making a horizontal measuring beam incident on a reference plane provided on the wafer stage. The reference plane is inclined by 45° and the vertically upwardly reflected light is reflected back by a horizontal mirror placed above such that the downwardly reflected light is again reflected on the reference plane to be returned to the sensor.
In the technical field of photolithography, it is known to split a single laser beam into a measurement and reference beams and combine them optimally to bring to a sensor and associated electronics for measurement of a relative displacement therebetween. U.S. Pat. No. 6,208,407 describes an arrangement with two height measuring beams, requiring interferometer optics and measurement beams on both sides of the stage. This makes it impractical for certain multistage systems where a second stage must pass the first stage on one side because one of the measurement beams incident on one stage will be blocked by the other stage.
It is desirable, therefore, to provide an interferometer system capable of measuring the height of a wafer stage even with small displacements to a first degree in other degrees of freedom and usable for a multistage system.